Board members of the Buena Vista Audubon Society announced this week they have purchased 31 acres of a former dairy farm between the edge of Camp Pendleton and the San Luis Rey River, near Whelan Lake bird sanctuary.

“The goal is to restore it to native habitat,” said Natalie Shapiro, board president of the Oceanside-based group. “There will never be any development on it. We can’t even build a bathroom … but maybe a fence and an information kiosk.”

The property, purchased for $1.56 million, is amid rolling hills about five miles from the coast and has a commanding view of the nearby river valley, she said.

“It’s a thicket of mustard, fennel and grasses, all invasive species,” Shapiro said. “The big deal for us is that the Defense Department paid for half the property costs and all the costs to remove the invasive (plants), and to plant and seed the natives.”

Most of the rest of the purchase costs were covered by a $700,000 grant from the state Natural Resources Agency and smaller donations from the California Audubon Society, North County Advocates, the Malk Nature Fund and a few individual donors. The Defense Department also will fund an endowment to oversee the perpetual monitoring and maintenance of the property.

About 90 percent of Southern California’s native coastal sage scrub habitat has been lost to development, Shapiro said. The purchase will create a protected swath of native plants and brush from Camp Pendleton to the river for rare and endangered species of birds such as the California gnatcatcher.

“The reason this property was important was that it provides such a strong link between the very long riparian riverbed and the hundreds of thousands of acres of native lands that still exist on Camp Pendleton right now,” said Andy Mauro, also a Buena Vista Audubon Society board member.

“It’s a nice fit for us,” Mauro said of the purchase.

The Audubon Society has led monthly bird walks at Whelan Lake for 20 years, and the city recently restored some contiguous native habitat as mitigation for clearing nonnative brush from the San Luis Rey River channel.

The land is part of what was once a large dairy farm founded by the Whelan family at the turn of the 20th century and operated through the 1980s. Some of the old dairy buildings remain on the adjacent 73-acre bird sanctuary, which is owned by a private foundation, and parts of the old farm also are now a city golf course and sewage treatment plant.

Whelan Lake was created to serve the family’s dairy farm. The lake is not open to the public, except by special arrangement with the Audubon Society, and is preserved as a sanctuary for resident and migratory waterfowl. More than 170 species of birds have been spotted there, including Canada geese, willow flycatchers, least Bell’s vireos and 15 different species of ducks.

The society’s recent purchase, known as the Cheatham property for its most recent owners, is the second piece of land recently acquired by the Buena Vista Audubon Society. The group has its headquarters and a nature center on property rented from the city of Oceanside at South Coast Highway on the edge of the Buena Vista Lagoon.

The Audubon Society announced in January 2016 it had purchased 3.5 acres along the lagoon west of South Coast Highway, also for $1.5 million, where a hotel had been proposed years earlier. The goal there is to include the property in the lagoon restoration effort headed by the San Diego Association of Governments.

“We just wanted to make sure there wasn’t a hotel built on it,” Shapiro said.

The 31 acres of Cheatham property is all that remains of land the family had initially purchased to subdivide for residential development. Houses were built on eight nearby lots in the early 2000, but development fizzled on the rest of the land.